The present invention refers to a process for producing cans with a polygonal, usually square, cross-section, having a tubular body with a longitudinal lateral seam.
There are known cans which have a cylindrical tubular body obtained by conventional operations of cutting the metallic sheet, calendering the sheet to a cylindrical tubular shape and longitudinally welding the sheet for laterally closing the body.
Aiming at increasing the structural resistance of the lateral wall of the can body, the latter is often submitted to an operation, usually in a milling machine, for providing the lateral wall with a certain number of circumferentially reinforcing ribs, which are axially spaced from each other and slightly projecting inwardly of the can body.
Said ribs are obtained by deformation of the cylindrical lateral wall of the can body, allowing to increase the structural resistance of the can in the radial direction and, consequently, to use a thinner plate for manufacturing the can, thus relevantly reducing the cost of the final product.
Nevertheless, the above mentioned constructive solution is only economically and industrially viable when applied to cylindrical cans, in which the ribs are circumferentially developed, without interruption and without weakening the structure of the can in the axial direction. In the cylindrical cans, the provision of the continuous circumferential ribs does not reduce the resistance of the can against axial compression forces in a degree sufficient to impair the normal operation of the can, being therefore possible to compensate the thickness reduction of the metallic sheet by providing said reinforcing ribs.
However, in the case of cans with a polygonal cross-section, more specifically the cans with a square section and with continuous round longitudinal edges, the provision of said continuous circumferential reinforcing ribs, in order to increase the resistance of the walls against radial forces and to allow a corresponding thickness reduction in the metallic sheet, has not proven to be convenient because said ribs weaken, in an unacceptable manner, the longitudinal edges of the can, which suffer a great reduction in their resistance against axial compression forces, impairing the operation of the can. The attempts to compensate the thickness reduction in the metallic sheet of cans with a square cross-section, by providing continuous circumferential ribs, have not reached a satisfactory result, due to the degree of weakness produced in the longitudinal edges of the can.
However, the interruption of the circumferential ribs in the region of the longitudinal edges of the can requires those ribs to be produced only on the lateral wall portions, by rather complex operations which lead to the production of undesirable wrinkles in the end regions of each rib extension. These inconveniences have prevented the cans with a polygonal, usually square cross-section, from having the thickness reduction of the metallic sheet compensated by the provision of circumferential reinforcing ribs.
Thus, it is an objective of the present invention to provide a process for manufacturiong a can with a polygonal, usually square cross-section, which allows, through simple operations, to provide said can with circumferential reinforcing ribs on the lateral walls, without provoking weakness in the longitudinal edges of the can and allowing to reduce the thickness of the metallic sheet which forms the lateral walls of the can.
A further objective of the present invention is to provide a can with a polygonal cross-section, whose tubular body is provided with circumferential reinforcing ribs, which are interrupted in the longitudinal edges of the can.
The process for manufacturing the can with a polygonal cross-section comprises the initial step of forming, from a metallic sheet, a tubular body having a cylindrical lateral wall with a longitudinal seam.
According to the invention, the manufacturing process further comprises the steps of:
milling the cylindrical lateral wall of the tubular body, in order to provide said wall with a plurality of circumferential ribs, which are continuous, axially spaced from each other and defined by a certain degree of radial plastic deformation of the respective circumferential region of the cylindrical lateral wall of the tubular body;
expanding the tubular body of the can to the desired polygonal cross-section, deforming its lateral wall in order to define longitudinal edges, in whose region the circumferential ribs are eliminated; and
double-seaming a bottom and an upper wall of the can to the tubular body of polygonal cross-section.
According to the new process, the steps for forming the can body are simple, do not require complex proceedings and are the same as those generally used, until the point of laterally closing the tubular body of the can. However, from this point on, the steps for forming the cans with a polygonal cross-section are distinct from those already known. The tubular body is then provided with the continuous circumferential ribs, which are eliminated in the longitudinal edges of the can during the step of expanding the tubular body from the original cylindrical shape to the desired polygonal tubular shape.
The dimensioning of the deformation degree of the lateral wall, in order to form the ribs, is effected in order to allow that, after the step of expanding the tubular body to the polygonal shape, the ribs are practically eliminated in the region of the longitudinal edges of the can, maintaining the structural integrity of the columns defined by said longitudinal edges, said ribs however being still maintained along the respective lateral wall portions of the tubular body, which is now polygonal, increasing the resistance of these lateral wall portions and allowing a substantial reduction in the thickness of the metallic sheet, without impairing the structure of the can.